Generally, the first indication of worsening heart failure is significant swelling or breathing difficulties that cause the patient to visit a clinic, or be transported to a hospital on an emergency basis. By this point, cardiac decompensation may have progressed to a point requiring patient hospitalization. Accordingly, a variety of medical devices have been used, or proposed for use, to monitor heart failure, detect heart failure events, and provide an alert to the patient to seek medical treatment prior to the onset of worsening heart failure with symptoms that require hospitalization, such as severe pulmonary edema. Typically, such medical devices have been implantable. In many cases, such devices have been cardiac pacemakers, cardioverters, and/or defibrillators with added heart failure monitoring functionality.
Some medical devices have monitored heart failure by monitoring intrathoracic impedance. Intrathoracic impedance may generally provide an indication of the level of edema in patients. Worsening heart failure may result in cardiac chamber dilation, increased interstitial fluid volume, and pulmonary edema—all of which contribute to a decrease in intrathoracic impedance.